Re: static linking and dlopen

[Paul Schutte <sjpschutte@gmail.com>]

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Thanks for the comprehensive answer, this is the answer I was looking for.

There is currently not a situation that require me to do this. I was just
thinking about it and decided to ask the experts.

On Sun, Dec 9, 2012 at 12:52 AM, Rich Felker <dalias@aerifal.cx> wrote:

> On Sat, Dec 08, 2012 at 08:09:35PM +0200, Paul Schutte wrote:
> > Hi,
> >
> > I have a strong preference towards static linking these days because the
> > running program use so much less memory.
> >
> > When I link a binary statically and that binary then use dlopen, would
> that
> > work 100% ?
>
> Presently, it does not work at all. At best, it loses all the
> advantages of static linking.
>
> > What would open if the shared object that was dlopened want's to call
> > functions in other shared libraries ?
>
> Dependencies of any loaded library also get loaded.
>
> > I understand that when using dynamic linking those libraries would just
> get
> > loaded, but I am not sure what would happen with static linking.
>
> With static linking, they would have to be loaded too. This means a
> static-linked program using dlopen would have to contain the entire
> dynamic linker logic. What's worse, it would also have to contain at
> least the entire libc, and if you were using static versions of any
> other library in the main program, and a loaded module referenced a
> dynamic version of the same library, you'd probably run into
> unpredictable crashing when the versions do not match exactly.
>
> The source of all these problems is basically the same as the benefit
> of static linking: the fact that the linker resolves, statically at
> link time, which object files are needed to satisfy the needs of the
> program. With dlopen, however, there is no static answer; *any* object
> is potentially-needed, not directly by the main program, but possibly
> by loaded modules. Consider what happens now if you only link part of
> libc into the main program statically: additional modules loaded at
> runtime won't necessarily have all the stuff they need, so dlopen
> would also have to load libc.so. But now you're potentially using two
> different versions of libc in the same program; if
> implementation-internal data structures like FILE or the pthread
> structure are not identical between the 2 versions, you'll hit an ABI
> incompatibility, despite the fact that these data structures were
> intended to be implementation-internal and never affect ABI. Even
> without that issue, you have issues like potentially 2 copies of
> malloc trying to manage the heap without being aware of one another,
> and thus clobbering it.
>
> For libc, the issues are all fixable by making sure that a static
> version of dlopen depends on every single function in libc, so that
> another copy never needs to get loaded. However, for other static
> libraries pulled into the main program, there is really no fix without
> help from the linker (it would have to pull in the entire library, and
> somehow leave a note for dlopen to see that library is already loaded
> and avoid loading it dynamically too).
>
> Note that, even if we could get this working with a reasonable level
> of robustness, almost all the advantages of static linking would be
> gone. Static-linked programs using dlopen would be huge and ugly.
>
> If you really want to make single-file binaries with no dependencies
> and dlopen support, I think the solution is to first build them
> dynamically linked, then merge the main program and all .so files into
> a single ELF file. I don't know of any tools capable of doing this,
> but in principle it's possible to write one. There are at least 2
> different approaches to this. One is to process the ELF files and
> merge their list of LOAD segments, symbol and relocation tables, etc.
> all into a single ELF file, leaving the relocations in place for the
> dynamic linker to perform at startup. This would require some
> modification to the dynamic linker still. The other approach is the
> equivalent of emacs' unexec dumper: place some kind of hook to run
> after the dynamic linker loads everything, but before any other
> application code runs, which dumps the entire memory space to an ELF
> file which, when run, will reconstruct itself.
>
> Rich
>

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